Methods for producing improved crystallinity group iii-nitride crystals from initial group iii-nitride seed by ammonothermal growth

ABSTRACT

The present invention discloses methods to create higher quality group III-nitride wafers that then generate improvements in the crystalline properties of ingots produced by ammonothermal growth from an initial defective seed. By obtaining future seeds from carefully chosen regions of an ingot produced on a bowed seed crystal, future ingot crystalline properties can be improved. Specifically, the future seeds are optimized if chosen from an area of relieved stress on a cracked ingot or from a carefully chosen N-polar compressed area. When the seeds are sliced out, miscut of 3-10° helps to improve structural quality of successive growth. Additionally a method is proposed to improve crystal quality by using the ammonothermal method to produce a series of ingots, each using a specifically oriented seed from the previous ingot. When employed, these methods enhance the quality of Group III nitride wafers and thus improve the efficiency of any subsequent device.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a Divisional of U.S. patent application Ser. No.14/192,715, filed Feb. 27, 2014, and entitled “Methods For ProducingImproved Crystallinity Group III-Nitride Crystals From Initial GroupIII-Nitride Seed By Ammonothermal Growth”, inventors Edward Letts, TadaoHashimoto, and Masanori Ikari, which is a Divisional of U.S. patentapplication Ser. No. 12/455,760, filed Jun. 4, 2009, and entitled“Methods For Producing Improved Crystallinity Group III-Nitride CrystalsFrom Initial Group III-Nitride Seed By Ammonothermal Growth”, with sameinventors, which claims the benefit of priority to U.S. ProvisionalPatent Application No. 61/058,900, filed Jun. 4, 2008, having sameinventors, and entitled “Methods For Producing Improved CrystallinityGroup III-Nitride Crystals From Initial Group III-Nitride Seed ByAmmonothermal Growth”. The entire contents of each of the foregoingapplications is incorporated by reference herein as if put forth in fullbelow. This application is also related to PCT Application No.PCT/US2009/046316, filed Jun. 4, 2009, and entitled “Methods ForProducing Improved Crystallinity Group III-Nitride Crystals From InitialGroup III-Nitride Seed By Ammonothermal Growth”, same inventors, theentire contents of which are incorporated by reference herein as if putforth in full below.

This application is related to the following U.S. patent applications:

PCT Utility Patent Application Serial No. US2005/024239, filed on Jul.8, 2005, by Kenji Fujito, Tadao Hashimoto and Shuji Nakamura, entitled“METHOD FOR GROWING GROUP III-NITRIDE CRYSTALS IN SUPERCRITICAL AMMONIAUSING AN AUTOCLAVE,” attorneys' docket number 30794.0129-WO-01(2005-339-1);

U.S. Utility patent application Ser. No. 11/784,339, filed on Apr. 6,2007, by Tadao Hashimoto, Makoto Saito, and Shuji Nakamura, entitled“METHOD FOR GROWING LARGE SURFACE AREA GALLIUM NITRIDE CRYSTALS INSUPERCRITICAL AMMONIA AND LARGE SURFACE AREA GALLIUM NITRIDE CRYSTALS,”attorneys docket number 30794.179-US-U1 (2006-204), which applicationclaims the benefit under 35 U.S.C. Section 119(e) of U.S. ProvisionalPatent Application Ser. No. 60/790,310, filed on Apr. 7, 2006, by TadaoHashimoto, Makoto Saito, and Shuji Nakamura, entitled “A METHOD FORGROWING LARGE SURFACE AREA GALLIUM NITRIDE CRYSTALS IN SUPERCRITICALAMMONIA AND LARGE SURFACE AREA GALLIUM NITRIDE CRYSTALS,” attorneysdocket number 30794.179-US-P1 (2006-204);

U.S. Utility Patent Application Ser. No. 60/973,602, filed on Sep. 19,2007, by Tadao Hashimoto and Shuji Nakamura, entitled “GALLIUM NITRIDEBULK CRYSTALS AND THEIR GROWTH METHOD,” attorneys docket number30794.244-US-P1 (2007-809-1);

U.S. Utility patent application Ser. No. 11/977,661, filed on Oct. 25,2007, by Tadao Hashimoto, entitled “METHOD FOR GROWING GROUP III-NITRIDECRYSTALS IN A MIXTURE OF SUPERCRITICAL AMMONIA AND NITROGEN, AND GROUPIII-NITRIDE CRYSTALS GROWN THEREBY,” attorneys docket number30794.253-US-U1 (2007-774-2);

U.S. Utility Patent Application Ser. No. 61/067,117, filed on Feb. 25,2008, by Tadao Hashimoto, Edward Letts, Masanori Ikari, entitled “METHODFOR PRODUCING GROUP III-NITRIDE WAFERS AND GROUP III-NITRIDE WAFERS,”attorneys docket number 62158-30002.00;

which applications are incorporated by reference herein.

BACKGROUND

1. Field of the Invention

The invention is related to the production method of group III-nitridewafers using the ammonothermal method combined with cutting andprocessing of an ingot to improve the crystal quality from an initialgroup III-nitride seed.

2. Description of the Existing Technology

(Note: This patent application refers to several publications andpatents as indicated with numbers within brackets, e.g., [x]. A list ofthese publications and patents can be found in the section entitled“References.”)

Gallium nitride (GaN) and its related group III alloys are the keymaterial for various opto-electronic and electronic devices such aslight emitting diodes (LEDs), laser diodes (LDs), microwave powertransistors, and solar-blind photo detectors. Currently LEDs are widelyused in cell phones, indicators, displays, and LDs are used in datastorage disc drives. However, the majority of these devices are grownepitaxially on heterogeneous substrates, such as sapphire and siliconcarbide. The heteroepitaxial growth of group III-nitride causes highlydefected or even cracked films, which hinders the realization ofhigh-end optical and electronic devices, such as high-brightness LEDsfor general lighting or high-power microwave transistors.

Most of the problems inherent in heteroepitaxial growth could be avoidedby instead using homoepitaxial growth with single crystalline groupIII-nitride wafers sliced from bulk group III-nitride crystal ingots forhomoepitaxy. For the majority of devices, single crystalline GaN wafersare favored because it is relatively easy to control the conductivity ofthe wafer and GaN wafers will provide the smallest lattice/thermalmismatch with device layers. Currently, however, the GaN wafers neededfor homogeneous growth are extremely expensive compared toheteroepitaxial substrates. This is because it has been difficult togrow group III-nitride crystal ingots due to their high melting pointand high nitrogen vapor pressure at high temperature. Growth methodsusing molten Ga, such as high-pressure high-temperature synthesis [1,2]and sodium flux [3,4], have been proposed to grow GaN crystals.Nevertheless the crystal shape grown using molten Ga is a thin plateletbecause molten Ga has low solubility of nitrogen and a low diffusioncoefficient of nitrogen.

An ammonothermal method, which is a solution growth method usinghigh-pressure ammonia as a solvent, has been used to achieve successfulgrowth of real bulk GaN ingots [5]. Ammonothermal growth has thepotential for growing large GaN crystal ingots because high-pressureammonia has advantages as a fluid medium including high solubility ofsource materials, such as GaN polycrystals or metallic Ga, and hightransport speed of dissolved precursors.

Currently, state-of-the-art ammonothermal method [6-8] relies on seedcrystals to produce large ingots. A lack of large seed crystals free ofstrains and defects limits the growth of high quality bulk GaN ingotswith a diameter of 3″ or greater. Several potential seeds produced bydifferent methods exist; however the seeds tend to be either small ordefective. For instance, 2″ free standing GaN wafers have been producedby the Hydride Vapor Phase Epitaxy (HYPE) on sapphire or SiC substrates.Due to the large lattice mismatch between GaN and the sapphire or SiCsubstrates, the resulting GaN growth is bowed, strained and has a largedefect density. Continued growth on a free standing seed produced byHVPE typically produces defective growth. In contrast, GaN crystalsproduced by the high pressure synthesis or sodium flux method tend tohave high quality but limited size and availability. A method to improvedefective seed crystals would improve the feasibility of producing largeingots suitable for use as substrates for devices.

SUMMARY OF THE INVENTION

To address the problems inherent with growth on the available defectiveseed, the present invention discloses a new growth scheme including 3different methods to improve the crystal quality of group III-nitridecrystals grown by the ammonothermal method. Due to the lattice mismatchof GaN and typical heteroepitaxial substrates, seed crystals produced byheteroepitaxial methods show concave bowing of c-plane lattice along +cdirection with typical curvature radius of 1 m. However, we discoveredthat subsequent growth of GaN by the ammonothermal method on such seedcrystals results in flipping over the bowing direction. Therefore, GaNon the Ga-polar (0001) surface grows under tensile stress while GaN onthe N-polar (000-1) surface grows under compression. The compression onthe N-polar surface prevents cracking and allows continuous orientedgrowth. Moreover, one can obtain very flat crystal by choosingappropriate growth thickness before the bowing direction flips. Aftergroup III-nitride ingots are grown by the ammonothermal method, theingots are sliced into wafers whose thickness is between about 0.1 mmand about 2 mm. By cutting from N-polar growth at the optimizedposition, orientation and miscut so that the cut surface is not alongthe crystal face but at an angle to the crystal face, the resultingwafer can be used as an improved seed for subsequent growths that willthen have limited bowing and reduced stress.

By comparison, growth on the Ga-polar surface tends to crack. Anothermethod to obtain a seed with lower strain and bowing is to harvest asmall crack free region on the Ga-polar (0001) face of an initial ingotin which cracking occurred. Cracking relieves the stress in thesurrounding region of growth. By harvesting one of these localizedregions of relieved stress as a seed crystal, subsequent ingot growthwould produce an improved crystal quality compared to the initial seedcrystal.

Lastly, a method is disclosed to produce seed crystal(s) with improvedcrystal quality from an initial seed crystal and can be achieved by thegrowth of a series of ingots each produced on a wafer with a specificcrystal orientation harvested from the previous ingot.

BRIEF DESCRIPTION OF THE DRAWINGS

Referring now to the drawings in which like reference numbers representcorresponding parts throughout:

FIG. 1 Primary crystallographic planes of the III:nitride wurtzitecrystal lattice. On the left is the historically used c-plane, on theright are the non-polar a-plane and m-plane.

FIG. 2 Photomicrograph of N-polar facet of GaN grown by theammonothermal method. No cracking was observed after 400 μm of growth onthe N-polar facet. The scale bar is equal to 100 μm.

FIG. 3 An exaggerated illustration of the bowing profile on a potentialseed crystal and the expected bowing profile of the resulting growth.

FIG. 4 Photomicroscope of Ga-polar facet of GaN grown by theammonothermal method. Cracking was observed after 400 μm of growth onthe Ga-polar facet. The scale bar is equal to 100 μm.

FIG. 5 An illustration of the seed's c-plane growth orientation before(left) and after (right) ammonothermal growth for the first ingot in theseries. The lines indicate the direction of the wires to sliceorientated wafers out of the ingot.

FIG. 6 An illustration of the seed's a-plane growth orientation before(left) and after (right) ammonothermal growth for the second ingot inthe series. The lines indicate the direction of the wires to sliceorientated wafers out of the ingot.

FIG. 7 An illustration of the seed's a-plane growth orientation before(left) and after (right) ammonothermal growth for the third ingot in theseries. The lines indicate the direction of the wires to sliceorientated wafers out of the ingot.

DETAILED DESCRIPTION OF THE INVENTION

In the following description of the preferred embodiment, reference ismade to the accompanying drawings which form a part hereof, and in whichis shown by way of illustration a specific embodiment in which theinvention may be practiced. It is to be understood that otherembodiments may be utilized and structural changes may be made withoutdeparting from the scope of the present invention.

Technical Description of the Invention

The present invention provides a method of producing group III-nitridewafers, primarily group III-nitride single crystalline wafers thatinclude at least one of the group III elements B, Al, Ga and In, such asGaN, AlN and InN. The group III-nitride ingots are grown by theammonothermal method which utilizes high-pressure NH₃ as a fluid medium,nutrient containing group III elements, and seed crystals that are groupIII-nitride single crystals. The high-pressure NH₃ provides highsolubility of the nutrient and high transport speed of dissolvedprecursors. After the group III-nitride ingots are grown, the ingots aresliced into wafers of thickness between about 0.1 mm and about 2 mmusing conventional means such as by mechanically sawing with a wire saw,a dicing saw, or by laser cutting. The III-nitride crystal structure ofinterest has a wurtzite crystal structure with the important facets c,m, and a-planes shown in FIG. 1.

In one instance, a method for growing group III nitride crystalsincludes:

(a) growing group III nitride ingots on original seed crystals by theammonothermal method;

(b) slicing wafers out of the ingots;

(c) using wafers taken from the nitrogen-polar side of the original seedcrystals as new seed crystals for subsequent growth of ingots by theammonothermal method.

The group III nitride may be GaN, for instance.

The original seed crystals may be formed using a heteroepitaxialdeposition process if desired.

The method may also include slicing new wafers from thesubsequently-grown ingots and using these new wafers as seeds insubsequent ammonothermal growth of new ingots.

The method may therefore be practiced under conditions that provide:

improvement in the bowing of crystallographic lattice along the slicingdirection as compared to the initial seed crystals;

strain in the new seed is reduced from the initial seed crystals;

crystallinity is improved over crystallinity of the initial seedcrystals;

bowing of the crystallographic lattice along the slicing direction isinverted from the initial seed crystal;

bowing of crystallographic lattice along the slicing direction isimproved over the initial seed crystals;

strain in the new seed is reduced from the initial seed crystals; and/orcrystallinity is improved from the initial seed crystals.

In any of the instances above, wafers may be sliced from the ingot in aplane misoriented from c plane of the grown crystals by 3 to 15 degrees.

The slice may be formed to provide:

bowing of crystallographic lattice along the slicing direction isimproved from the initial seed crystals;

strain in the new seed is reduced from the initial seed crystals; and/or

crystallinity is improved from the initial seed crystals.

An additional method for growing group III nitride crystals involves:

(a) growing group III-nitride ingots on original seed crystals by theammonothermal method until some cracking occurs;

(b) separating a crack free region out of the ingots; and

(c) using the separated region as a new seed for subsequent growth of aningot.

The group III nitride may be e.g. GaN.

The original seed crystals may optionally be formed using aheteroepitaxial deposition process for forming group III-nitridecrystals such as GaN.

The method may additionally include slicing new wafers from thesubsequently-grown ingots and using these new wafers as seeds in asubsequent ammonothermal growth of new ingots.

Any of these methods may be performed under conditions wherein:

the bowing of crystallographic lattice along the slicing direction isimproved from the initial seed crystals;

strain in the new seed is reduced from strain in the initial seedcrystals; and/or

crystallinity is improved over the crystallinity of the initial seedcrystals.

Wafers may be sliced from the ingot along a plane misoriented from cplane by 3 to 15 degrees, and optionally the wafers may be used as newseed material in ammonothermal growth of new ingots.

A third method of growing group III-nitride crystals may include:

(a) growing ingots (e.g. a first ingot) on c-facets of seed crystals(e.g. a first seed crystal) by the ammonothermal method to a thicknessgreater than 5 mm;

(b) slicing the ingots (e.g. the first ingot) along the a-plane or asemi-polar plane to form seeds (e.g. a second seed crystal);

(c) using the a-plane or semi-polar plane seeds (e.g. the second seedcrystal) to grow new ingots (e.g. a second ingot);

(d) slicing the new ingots (e.g. the second ingot) along the a-plane orthe semi-polar plane; and

(e) using a-plane or semi-polar plane wafers not containing any originalmaterial of the initial seed crystal (e.g. third seed crystal) to growadditional new ingots (e.g. a third ingot).

The method may be practiced using only a-plane slices or only semi-polarplane slices, or the method may be performed by using one slicingdirection for one ingot and another slicing direction for a subsequentingot.

The group III-nitride may be e.g. GaN.

The method in any of these instances may further include slicing aningot obtained in step (e) above to produce c-plane wafers.

The method may be performed under conditions where:

bowing of crystallographic lattice along the slicing direction isimproved from the initial seed crystals;

strain in the new seed is reduced from the initial seed crystals; and/or

crystallinity is improved from the initial seed crystals.

GaN wafers may be produced in which the c-plane lattice bows convexly inthe +c direction.

These GaN wafers may have a basal plane that is c-plane and miscutwithin 10 degrees.

The GaN wafers may have a basal plane that is m-plane and miscut within10 degrees.

The GaN wafers may have a basal plane that is a-plane and miscut within10 degrees.

The following additional detailed explanation describes detailedprocedures to aid in further understanding of the invention.

Method 1

A reaction vessel with an inner diameter of 1 inch was used for theammonothermal growth. All necessary sources and internal components wereloaded together with the reaction vessel into a glove box. In one growthoccasion, these components included 10 g of polycrystalline GaN nutrientheld in a Ni mesh basket, 0.34 mm-thick single crystalline c-plane GaNseeds, and six baffles to restrict flow. The initial GaN seed wasproduced by HVPE on sapphire which caused the seed crystal to be bowedand strained. The glove box is filled with nitrogen, and the oxygen andmoisture concentration was maintained at less than 1 ppm. Since themineralizers are reactive with oxygen and moisture, the mineralizerswere stored in the glove box all the time. 4 g of as-received NaNH₂ wasused as a mineralizer. After loading mineralizer into the reactionvessel, six baffles together with seeds and nutrient were loaded. Afterclosing the lid of the reaction vessel, the reaction vessel was takenout of the glove box. Then, the reaction vessel was connected to agas/vacuum system, which can pump down the vessel as well as can supplyNH₃ to the vessel. First, the reaction vessel was evacuated with a turbomolecular pump to achieve a pressure of less than 1×10⁻⁵ mbar. Theactual pressure achieved for this example was 1.2×10⁻⁶ mbar. In thisway, residual oxygen and moisture on the inner wall of the reactionvessel were partially removed. After this, the reaction vessel waschilled with liquid nitrogen and NH₃ was condensed in the reactionvessel. About 40 g of NH₃ was charged in the reaction vessel. Afterclosing the high-pressure valve of the reaction vessel, the reactionvessel was transferred to a two zone furnace. The reaction vessel washeated to 510° C. in the crystallization zone and 550° C. in thedissolution zone for the first 24 hrs before being to adjusted to 575°C. in the crystallization zone and 510° C. in the dissolution zone.After 8 days, ammonia was released and the reaction vessel was opened.The total thickness of the grown GaN ingot was 1.30 mm.

Microscope images of the growth on the Ga-polar surface showed crackingwhile the N-polar surface showed no cracking and a relatively flatsurface, see FIG. 2. Crystal structure measured on the N-polar surfaceshowed a single peak from 002 reflection. The Full Width Half Max (FWHM)of the peak was 209 arcsecs. On the other hand, the Ga-polar surfaceshowed multiple sharp peaks from 002 reflections with FWHM of 2740arcsec. The multiple sharp peaks from Ga-polar side represent agathering of high-quality grains. This difference in growth on thedifferent polarities is caused by the bowing of the seed crystal, asdiagrammed in FIG. 3. Bowing of the seed crystal causes the growth onthe Ga-polar surface to be under tensile strain and prone to crackingwhile the growth on the N-polar surface is under compressive strainwhich prevents cracking of the growth.

The bowing profile was improved in the N-polar growth compared to theinitial seed bowing profile, as shown in FIG. 3. In one growth occasion,the radius of lattice bowing on N-polar side was improved to 130 m(convex) from 1.15 m (convex), which was the original radius of latticebowing of the seed.

By harvesting the N-polar growth as a seed for future ingots, problemsassociated with bowing may be minimized allowing subsequent crack freegrowth on the Ga-polar surface as well. In addition, optimization of thegrowth thickness should yield improved crystallinity for future ingots.

It was also confirmed that using miscut substrates as seed crystalshelps to improve crystal quality. In one growth occasion, ammonothermalgrowth was conducted with two kinds of miscut seeds, one with 7° offfrom the c-plane and the other with 3° off from the c-plane. The FWHM ofX-ray rocking curve from 002 reflection of the original seeds were 605arcsec and 405 arcsec for 7° off and 3° off, respectively. After growth,the FWHM of X-ray rocking curve became 410 arcsec and 588 arcsec for 7°off and 3° off, respectively. From this result, it was confirmed thatmiscut as much as approximately 7° helps improve structural quality.Miscut could be up to 10° or 15° off axis rather than up to 3° or up to7° off axis.

Method 2

With similar growth condition as indicated for Method 1, a GaN ingot asthick as 1.3 mm was obtained after 8 day-growth. Microscope images ofthe growth on the Ga Polar surface showed cracking as shown in FIG. 4.while the N-polar surface showed no cracking and a relatively flatsurface. As explained for Method 1, the crystal on Ga-polar sideconsists of many high-quality grains. Therefore, it is expected thatafter cracking occurs, harvesting a relaxed region of the growth on theGa-polar surface as a seed crystal would enable future ingots to exhibitimproved crystallinity from the initial seed crystal. Harvested regionsare expected to have a Ga-polar surface area between about 0.1 mm² andabout 5.0 mm².

Method 3

The ammonothermal growth technique discussed above can be used toproduce a series of ingots and by selecting specific regions with acrystallographic orientations for subsequent seeds, the crystallinity ofIII-nitride material can be improved. Starting with an imperfect c-planeseed crystal, the first ingot primary growth direction is along thec-axis, as shown in FIG. 5. Due to cracking problems the growth on theGa-polar surface may not be suitable for continued growth. The firstingot is then sliced using a wire saw to produce a-plane wafers. Usingan a-plane wafer as a seed, a new ingot is then produced by theammonothermal growth techniques as shown in FIG. 6. The second ingot isthen sliced using a wire saw to produce a-plane wafers. By choosing awafer which contains no initial seed crystal as the new seed, a thirdingot can be produced which contains none of the initial seed crystal,as shown in FIG. 7. This third ingot can then be sliced with a wire sawin any given orientation to produce seed crystals of improvedcrystallinity.

This method promotes growth by limiting the size and effect of thedislocations, bowing, and strain of the seed. This method realizes bulkcrystal growth with very low threading dislocations densities and animproved bowing profile. This method can be modified to use a semipolaror m-plane growth instead of the a-plane orientation.

Advantages and Improvements

The present invention disclosed new production methods of groupIII-nitride wafers with improved crystal structure. Using severalpossible strategies, specific regions of a grown ingot may be harvestedas a future seed to drastically improve the quality of future ingotscompared to the initial seed. Additionally, a method is proposed toproduce a series of ingots that could produce an drastic improvement ofcrystalline quality. These improvement would improve efficiencies forany optical devices fabricated on the wafers.

REFERENCES

The following references are incorporated by reference herein:

-   [1]. S. Porowski, MRS Internet Journal of Nitride Semiconductor,    Res. 4S1, (1999) G1.3.-   [2] T. Inoue, Y. Seki, O. Oda, S. Kurai, Y. Yamada, and T. Taguchi,    Phys. Stat. Sol. (b), 223 (2001) p. 15.-   [3] M. Aoki, H. Yamane, M. Shimada, S. Sarayama, and F. J.    DiSalvo, J. Cryst. Growth 242 (2002) p. 70.-   [4] T. Iwahashi, F. Kawamura, M. Morishita, Y. Kai, M. Yoshimura, Y.    Mori, and T. Sasaki, J. Cryst Growth 253 (2003) p. 1.-   [5] T. Hashimoto, F. Wu, J. S. Speck, S. Nakamura, Jpn. J. Appl.    Phys. 46 (2007) L889.-   [6] R. Dwiliński, R. Doradziński, J. Garczyński, L.    Sierzputowski, Y. Kanbara, U.S. Pat. No. 6,656,615.-   [7] K. Fujito, T. Hashimoto, S. Nakamura, International Patent    Application No. PCT/US2005/024239, WO07008198.-   [8] T. Hashimoto, M. Saito, S. Nakamura, International Patent    Application No. PCT/US2007/008743, WO07117689. See US20070234946,    U.S. application Ser. No. 11/784,339 filed Apr. 6, 2007.

Each of the references above is incorporated by reference in itsentirety as if put forth in full herein, and particularly with respectto description of methods of growth using ammonothermal methods andusing gallium nitride substrates.

CONCLUSION

This concludes the description of the preferred embodiment of theinvention. The following describes some alternative embodiments foraccomplishing the present invention.

Although the preferred embodiment describes the growth of GaN as anexample, other group III-nitride crystals may be used in the presentinvention. The group III-nitride materials may include at least one ofthe group III elements B, Al, Ga, and In.

In the preferred embodiment specific growth apparatuses and slicingapparatus are presented. However, other constructions or designs thatfulfill the conditions described herein will have the same benefit asthese examples.

The present invention does not have any limitations on the size of thewafer, so long as the same benefits can be obtained.

The foregoing description of the preferred embodiment of the inventionhas been presented for the purposes of illustration and description. Itis not intended to be exhaustive or to limit the invention to theprecise form disclosed. Many modifications and variations are possiblein light of the above teaching. It is intended that the scope of theinvention be limited not by this detailed description, but rather by theclaims appended hereto.

What is claimed is:
 1. A method of fabricating a group III nitride wafercomprising: a. growing a first ingot of group III nitride on a firstseed crystal of group III nitride by an ammonothermal method until i. aradius of curvature of a c-plane lattice of the first ingot becomeslarger, and ii. the radius of curvature of the c-plane lattice of thefirst ingot inverts from a bowing direction of the first seed crystalalong the c-plane lattice of the first seed crystal; and b. slicing awafer out of the first ingot and from a nitrogen polar side of the firstingot.
 2. The method of claim 1, wherein the wafer has a latticecurvature that is convex toward the +c direction.
 3. The method of claim2, wherein the act of slicing the wafer comprising slicing a c-planeoriented wafer with miscut angle within 10 degrees.
 4. The method ofclaim 1, wherein the first ingot has a N-polar face during growth, andthe N-polar face of the first ingot grows under compression.
 5. Themethod of claim 4, wherein the compression provides continuous orientedgrowth.
 6. The method of claim 4, wherein the compression preventscracking of the first ingot at the N-polar face during growth of thefirst ingot.
 7. The method of claim 6, wherein the group III nitride ofthe first ingot is GaN, and the group III nitride of the first seedcrystal is GaN.
 8. The method of claim 7, wherein the wafer has alattice curvature convex toward the +c direction.
 9. The method of claim8, wherein the act of slicing the wafer comprising slicing a c-planeoriented wafer with miscut angle within 10 degrees.
 10. The method ofclaim 1, wherein the wafer is formed as a second seed crystal for growthof a second ingot of group III nitride.
 11. The method of claim 10,wherein the wafer has a lattice curvature that is convex toward the +cdirection.
 12. The method of claim 11, wherein the act of slicing thewafer comprising slicing a c-plane oriented wafer with miscut anglewithin 10 degrees.
 13. The method of claim 1, wherein strain in thewafer is reduced from strain in the first seed crystal.
 14. The methodof claim 13, wherein the wafer has a lattice curvature that is convextoward the +c direction.
 15. The method of claim 14, wherein the act ofslicing the wafer comprising slicing a c-plane oriented wafer withmiscut angle within 10 degrees.
 16. The method of claim 1, wherein thewafer has a thickness between about 0.1 mm and about 2 mm.
 17. Themethod of claim 1, wherein the act of slicing the wafer comprisesslicing multiple wafers out of the first ingot.
 18. The method of claim17, wherein at least one of said wafers has a lattice curvature that isconvex toward the +c direction.
 19. The method of claim 18, wherein saidmultiple wafers are sliced from the ingot with miscut angle within 10degrees.
 20. The method of claim 1, wherein the act of slicing the wafercomprising slicing a c-plane oriented wafer with miscut angle within 10degrees.
 21. A wafer formed by the method of claim
 2. 22. A wafer formedby the method of claim
 3. 23. A wafer formed by the method of claim 8.24. A wafer formed by the method of claim
 9. 25. A wafer formed by themethod of claim
 11. 26. A wafer formed by the method of claim
 12. 27. Awafer formed by the method of claim
 14. 28. A wafer formed by the methodof claim
 15. 29. A plurality of wafers formed by the method of claim 18.30. A plurality of wafers formed by the method of claim 19.